boot: Direct-XIP: Add check for IMAGE_F_ROM_FIXED flag

The commit adds logic that checks if image, that has IMAGE_F_ROM_FIXED
flag set together with proper address in header, has not been mismatched
with slot.
This code will prevent attempting to boot image that has been build for
different ROM address than a slot it currently resides in.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>